In addition to dietary effects on the composition of the fat of fish, 

 Lovern (1951) has noted specific modifications of deposited fat by small 

 groups of closely related species. This situation is not unlike that 

 found xn land animals in which the nature of the dietary fat is super- 

 xmposed upon a characteristic fat deposited by the species. Two types 

 of changes may occur before deposition: (1) selective hydrogenation or 

 dehydrogenation and (2) alteration of the chain length of one or more 

 fatty acids by shortening or lengthening processes. 



A physiologically reversible process of saturation and desaturation 

 of fats is known to occur in mammals, as demonstrated by feeding experi- 

 ments with mice in which saturated and unsaturated fatty acids containing 

 deuterium were used (Rittenberg and Schoenheimer 1937 and Schoenheimer 

 and Rittenberg 1936). For fish, a detailed fatty acid analysis of herring 

 made over periods of fasting, feeding, and spawning was compared to the 

 fat of Calanus finmarchicus , the principal organism in the herring diet 

 (Lovern I9I?1)« ty following changes in the degree of unsaturation of any 

 individual fatty acid it was observed, for the most part, that the com- 

 position of herring fat tended to approach that of Calanus finmarchicus 

 during the period of active feeding but gradually returned to the com- 

 position characteristic of the fasting state. The results of these stud- 

 ies indicate that fish probably possess enzyme systems capable of selective 

 hydrogenation or dehydrogenation of different groups of fatty acids simi- 

 lar to those systems found in mammals. 



Evidence of another specific modification of ingested fat by fish 

 and by other aquatic animals — that of shortening or lengthening processes 

 affecting fatty acids— also is based on studies of composition (Lovern 

 1951). Since the fats of most marine fish are made up of roughly equal 

 amounts of 16-, 18-, and 20-carbon-atom acids, decreased amounts of lU- 

 and 22-carbon-atom acids, and little or no 2U-carbon-atom acids, it is 

 possible to define an "average" composition. Large deviations from this 

 "average" distribution of fatty acids appear to be an indication of a 

 progressive shifting or alteration of some fatty acids, two carbon atoms 

 at a time, into others of different length. Examples of such transforma- 

 tions are found in the toothed whale (Lovern 19U2), in which chain short- 

 ening occurs, and in the liver fats of sharks and related fishes, in 

 which chain lengthening occurs. Another example is found in the liver 

 fatty acids of the catfish ( Anarrhicas lupus ), which bear evidence of 

 simultaneous chain lengthening and chain shortening (Lovern 1937) • 



Ingested fats are absorbed either directly in a finely divided e- 

 mulsified form or as the free fatty acids after hydrolysis by lipases. 

 The effect of lipolysis on absorption of fat in fish is not knownj how- 

 ever, fish do possess very active lipases in their gastro-intestinal 

 tract. As was shown by Lovern and Morton (1939)> rapid post-mortem build- 

 up of free fatty acids from lipids in the intestinal tract of halibut 

 occurs, the content of free fatty acid rising to over 20 percent in a 

 period of 5 to 10 minutes. 



Lipases are found rather widespread throughout other tissues and 

 organs of fish, as in other forms of life. Falk et al. (1927) and Noyes 

 et al. (1927) obtained extracts of whole trout and trout eggs that possess 



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